Every once in a blue moon, the following hack comes in handy, and I have to look it up again. I don’t remember where I first saw this technique in the wild, but I remember being blown away when I realized what was going on.
Effect
A grizzled neckbeard hands you a shell script, and tells you to run it, and that it will install the application you’ve been wanting to try out.
“What?” you say. “Does it download it from the Internet?”
“No. No network required.”
“What, do I run this with Java, or something?”
“No, it is self-contained.”
“Do I need to type a cryptic command sequence to get this to run?”
Look, please just run the script. It will install your application.”
Then you look at the file, and see that it is several megabytes long. And, sure enough, it installs your shiny new app. But how?
Preparation
You will need:
- a
tared andgzipped file containing the source or binary distribution you want to bundle - knowledge of which additional commands to run, if any, to compile, install or otherwise deal with the unpacked files
- an open mind
Method
You will be creating a self-extracting executable. The fact that the ‘executable’ portion is really interpreted shell script, is irrelevant. This technique hinges on the ability of the shell script to reference itself, ignore the shell script portion, and feed the remainder to the shell script as binary data:
I actually don’t prefer to use this version above, but the magic is easy to spot on line 3: tail -n +8 $0: $0 is a reference to this, the current script file, so this command dumps line 8 through the end of the file through a pipeline to a tar extraction command. A careful count of the source reveals that line 8 is where the binary tar data was appended.
I don’t like this version because you have to manually and carefully count the lines of code in your script and change line 3 accordingly. I much prefer the following snippet which is more robust, but perhaps a bit harder to follow:
This version uses file redirection on $0 to put the contents of the script through the whole shell block between curly braces, and uses a while read loop to pre-emptively throw away the lines of STDIN that correspond to the script preamble, before passing the remaining, binary data through tar. The while loop also cleverly uses the string matching capability of case to treat the exit 0 as a sentinel value. Thus, exit 0 serves as both the start-of-data marker, and the end-of-script command. Wild.
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